In 10 heart transplanted subjects (HTS) undergoing conventional immunosuppressive cyclosporine therapy, in comparison with 10 normal subjects, the 24 hour patterns of T lymphocyte subpopulations, namely, OKT3 (total T lymphocytes), OKT4 (helper lymphocytes) and OKT8 (cytotoxic or suppressor) in relation to the circadian rhythms for plasma cortisol (marker rhythm) and to circulating levels of cyclosporine were studied. From the collected data, it can be deduced that the OKT3, OKT4, OKT8 subpopulations and the plasma cortisol level show 24-hour non-periodic variations. The lymphocyte subpopulations show a negative correlation with circulating levels of cyclosporine. The negative correlation is "selective" and "delayed" in that it is detectable at particular and non-coinciding hours. Plasma cortisol is also negatively correlated to plasma cyclosporine. Assessing the meaning of the lack of a circadian rhythm of the lymphocyte subpopulation in HTS undergoing conventional cyclosporine therapy, and taking into account the pharmacological time-stage dependency, we can emphasize the idea that the optimization of anti-rejection therapy with cyclosporine may and should be performed as a time-modulated treatment.